Many illnesses have been related to the Indoor Air Quality, and are also known to have a direct impact on the productivity of a worker. Rising need for increased ventilation rates and controlled humidity level for improved Indoor Air Quality have thrown up both challenges and opportunities in the design of HVAC (heating, ventilating, and air conditioning) systems.
In HVAC and industrial processes, management of air and gases entails management of several of its' properties like temperature, humidity, particulate and gaseous contaminants, noise, etc.
There is now universal recognition of the importance to control humidity in controlled spaces especially in facilities with high occupancy rates such as schools, hospitals, cinema halls etc. Desiccant wheel based dehumdifiers are being increasingly used to control humidity in such applications.
One of the predominantly emerging approach to HVAC system design, to cater to large(r) ventilation rates, is to provide a dedicated outdoor air system, as a fresh air unit, to cater for the total fresh air load as well as to provide the necessary moisture removal for meeting the dehumidification (room internal latent) needs, and a separate unit or system is applied as a parallel unit to take care of the internal sensible load of the room or occupied space.
Thus there is a need to develop a variety of DOASs (Dedicated Outdoor Air Systems) for handling the fresh air and room's latent load through a fresh air unit. Some of the DOAS units or systems are purely based on use of mechanical refrigeration for both cooling and dehumidification, and there are others which, in combination with mechanical refrigeration/cooling, utilize either a variety of energy recovery wheels, singularly or in plurality, and in some instances also desiccant wheels which are either “passive” or “thermally activated”.
Dehumidification can be carried out by either mechanical refrigeration or by using a desiccant dehumidifier which employs a desiccant material to produce the dehumidification effect. Desiccant materials have a high affinity for water vapor. An example of a commonly used desiccant material is silica gel. Typically their moisture content (moisture holding capacity) is a function of the relative humidity of the surrounding air.
The most commonly used adsorbents broadly are:                Synthetic zeolites/molecular sieves        Activated aluminas        Silica gel/metal silicates:        
Adsorbents are granular, beaded, powder, or in several other forms e.g. cast, extruded, honeycomb matrix etc.
Para 5, column 1, pg 32.4, chapter 32 of 2009 ASHRAE Handbook—Fundamentals (SI) states as follows:
“Adsorption behaviour depends on (1) total surface area, (2) total volume of capillaries, and (3) range of capillary diameters. A large surface area gives the adsorbent a larger capacity at low relative humidities. Large capillaries provide a high capacity for condensed water, which gives the adsorbent a higher capacity at high relative humidities. A narrow range of capillary diameters makes an adsorbent more selective in the vapor molecules it can hold.
In designing a desiccant, some tradeoffs are necessary. For example, materials with large capillaries necessarily have a smaller surface area per unit of volume than those with smaller capillaries. As a result adsorbent are sometimes combined to provide a high adsorption capacity across a wide range of operating conditions. FIG. 5(b) illustrates this point using three noncommercial silica gel adsorbents prepared for use in laboratory research. Each has a different internal structure, but because they are all silicas, they have similar surface adsorption characteristics. Gel 1 has large capillaries, making its total volume large but its total surface area small. It has a large adsorption capacity at high relative humidities but adsorbs a small amount at low relative humidities.
In contrast, Gel 8 has a capillary volume one-seventh the size of Gel 1, but a total surface area almost twice as large. This gives it a higher capacity at low relative humidities but a lower capacity to hold the moisture that condenses at high relative humidities.
Silica gels and most other adsorbents can be manufactured to provide optimum performance in a specific application, balancing capacity against strength, mass, and other favorable characteristics.”
Thus adsorption behaviour depends on total surface area and pore volume. Most of the prior art is related to desiccant matrixes that use desiccants which are microporous or of type I isotherm.
Dehumidification is considered as a key feature of HVAC systems for thermal comfort. When desiccant dehumidification is used for management and treatment of air at atmospheric pressure, mainly honeycomb type of matrixes are used in order to maximize the surface area in contact with air passing through/over the desiccant, and also minimize the use of desiccant as well as minimize the pressure drop of the air across the “desiccant bed”.
The honeycomb matrixes can be formed using a variety of substrates like plastic sheet, metal/aluminum foil, organic and/or inorganic fiber substrates which are “paper” like, which at times can be quite porous. Depending upon the substrate of choice, the amount of desiccant to be “deposited/loaded” and the temperature at which the air/matrix will be working, following are some of the methods for deposition/loading the desiccant on the substrate to prepare the matrix.                a. coating        b. impregranation        c. in-situ synthesization        
While “coating” or “impregnating” choice can be made from the variety of desiccant powders from various types I to V [FIG. 5(a)], as these are produced in bulk and for a variety of industrial applications other than HVAC air treatment, in-situ synthesization of desiccants, of the types: silica gels, and metal silicates, in the porosity of the porous inorganic fiber substrate, and formed into a honeycomb have been mainly for industrial and commercial desiccant dehumidifier applications, where the desiccant wheels are invariably thermally (re)activated, with air at elevated temperature, ranging from 60° C.˜200° C.
As stated earlier above, some of the Dedicated Outdoor Air Systems (DOAS) units or systems are purely based on use of mechanical refrigeration for both cooling and dehumidification, and there are others which, in combination with mechanical refrigeration/cooling, utilize either a variety of energy recovery wheels, singularly or in plurality, and in some instances also desiccant wheels which are either “passive” or “thermally activated”.
In the variety of DOASs, and other HVAC equipment, units configured with “passive” dehumidification wheels are gaining ground. The “passive” dehumidification wheels, have only recently begun to be applied. The “passive” desiccant wheels, as the name suggests, are wheels which are not thermally activated i.e. do not use any heat for regeneration. To understand this better please refer to FIG. 1a which shows typical thermally activated wheels.
As depicted in FIG. 1(b), in a rotating desiccant wheel, typically there are two sectors: process sector (2) and the reactivation sector (3), through which the honeycomb matrix or beds move/rotate. In the process sector, air or gas is dried, or moisture removed, which is picked up by the rotating wheel/bed. In the reactivation sector, this desiccant wheel mass/matrix is exposed to an air stream (8) which is elevated in temperature, which drives the moisture out of the desiccant, which is removed, on a continuous basis. When the air is heated and surrounds the desiccant mass/matrix in the reactivation sector, it is the vapour pressure difference in the desiccant mass and that of the air surrounding it which determines the amount of, and the rate at which, moisture is given up by the desiccant matrix/mass.
In thermally activated wheels, the regeneration air stream is typically elevated to temperatures ranging between 60° C. to 200° C., depending upon a variety of factors, including, but not limited to, the choice of desiccant, sectoral division between process and reactivation sectors, bed rotational speed, etc. These thermally activated wheels help in achieving “deep” dehumidification, and so far, all the development of honeycomb matrix based desiccant wheels has focused on maximizing moisture removal, at air inlet conditions which are normally below or between 10˜50 grains/lbs (1.5 to 7 gms/kg), but can sometimes be as high as ambient moisture. Though desiccant coated or desiccant impregnated wheels have also been applied/used for this thermally activated wheel application, predominantly the development, and application, and use has been of honeycomb matrix in which the desiccant is synthesized “in-situ”.
In the development of all these “in situ” synthesized desiccant wheels, the focus has been to minimize the desiccant pore size and maximize/optimize its surface (pore) area to obtain “deep dehumidification”. Such desiccants created “in situ” are often referred to as type I desiccant in which the majority of the pore sizes are distributed between 15 to 40 Å, and more specifically close to 20 Å. These are having the type I isotherm as shown in FIG. 5(c).
U.S. Pat. No. 4,886,769 relates to a method of manufacturing a microporous dehumidifier element which has a differential adsorption of about 10% at RH>10%. The said patent discloses an element (with sufficient physical strength) for adsorbing an ultra-low concentration gas which is obtained by using synthesized zeolite powder dispersed in an aqueous solution of water glass.
U.S. Pat. No. 4,911,775 relates to a method of manufacturing a honeycomb type dehumidifying element which has an adsorption capacity limited to 40-45% at RH>90%.
U.S. Pat. No. 4,871,607 relates to a humidity exchanger element which has excellent heat resistance without any possibility of deterioration in the temperature of not less than 100° C. This humidity exchanger has a limited adsorption capacity.
U.S. Pat. No. 5,254,195 relates to a process for preparing a moisture exchange element wherein the amount of adsorbent deposited on the surface of the substrate is increased by impregnating the substrate with colloidal silica in addition to metal salt and acid.
U.S. Pat. No. 5,435,958 discloses a process for manufacturing a humidity exchange element wherein the honeycomb matrix is soaked in an acidic solution containing at least a titanium inorganic salt to convert said sodium silicate water glass to a titanium-containing silicate hydrogel. This results in humidity exchange element with improved moisture adsorbing capacity and requires less energy to regenerate the matrix.
U.S. Pat. No. 5,683,532 relates to a method of manufacturing an active silica gel honeycomb adsorbent body for dehumidification which has high efficiency of dehumidification and small passing resistance of gas in small channels. The honeycomb structure is fired with air containing reduced oxygen at 500° C. to remove organic components in paper.
U.S. Pat. No. 6,187,381 discloses a process for manufacturing a dehumidifying element wherein the honeycomb is immersed in silica sol and dried followed by immersion alkali silicate (20-35 wt %) and alkali hydroxide (20-50 wt %). The ratio of silicon oxide to alkali oxide should not exceed 10. In the said patent the moisture adsorption at 90% RH is 16.5% only.
U.S. Pat. No. 6,344,073 relates to a dehumidifying element and process for preparing the same. The dehumidifying material comprises silica gel and metal oxide. This dehumidifying element can be utilized mainly at medium humidity conditions.
U.S. Pat. No. 6,630,206 discloses a method for manufacturing a dehumidifying element wherein molecular sieve is immersed into the water glass solution. The differential of water adsorption between 50 and 100% RH is less than 40%.
Thus it can be seen that the prior art employs mainly honeycomb matrix comprising microporous desiccant for dehumidification. Such a desiccant has the advantage that it provides larger surface area for adsorption of moisture but suffers from the disadvantage of small pore volume and hence can be utilized only in medium to low humidity conditions, which has been, hitherto, the focus of desiccant dehumidification.
Further, none of the dehumidification systems in the prior art provides high moisture adsorption capacity between 50 and 100% relative humidities. In fact the water adsorption (capacity) differential between 50 and 100% relative humidity for all the above patents stands at an amount less than 40%.
In some of the prior art showing configurations where “passive” dehumidification wheels were used, the desiccant is deposited on the substrate by coating or impregnation. However, said dehumidification wheels suffer from the disadvantage that binders were essential for impregnation and coating which reduced the efficacy of the wheel by masking the desiccant performance. Further, when the desiccant encountered saturated air, there is a tendency for the desiccant to be washed out with time.
Air to be treated, particularly outside air, other than water vapor, can contain several gaseous contaminants, e.g. VOCs, odors etc., and it would be desirable to remove these through the desiccant matrix. Air to be treated, particularly when it is pre cooled to near saturation, such gaseous contaminants are sometime water soluble and together are condensed in the macro porous desiccant mainly through capillary adsorption. In the prior art, microporous desiccants are mainly used, which exhibit limited capillary adsorption and therefore limited adsorption of gaseous contaminants.
Accordingly, to overcome the problems encountered in the prior art, the inventors of the present invention provide a honeycomb matrix comprising a macroporous desiccant prepared “in-situ” for use in “passive”/“active” dehumidification, and also as a chemical filter, as described herein below.